Liquid Crystal Displays (LCDs) are widely used to display information. LCDs are used for direct view displays, as well as for projection type displays. The electro-optical mode, which is employed for most displays, still is the twisted nematic (TN)-mode with its various modifications. Besides this mode, the super twisted nematic (STN)-mode and more recently the optically compensated bend (OCB)-mode and the electrically controlled birefringence (ECB)-mode with their various modifications, as e.g. the vertically aligned nematic (VAN), the patterned ITO vertically aligned nematic (PVA)-, the polymer stabilized vertically aligned nematic (PSVA)-mode and the multi domain vertically aligned nematic (MVA)-mode, as well as others, have been increasingly used. All these modes use an electrical field, which is substantially perpendicular to the substrates, respectively to the liquid crystal layer. Besides these modes there are also electro-optical modes employing an electrical field substantially parallel to the substrates, respectively the liquid crystal layer, like e.g. the In Plane Switching (short IPS) mode (as disclosed e.g. in DE 40 00 451 and EP 0 588 568) and the Fringe Field Switching (FFS) mode. Especially the latter mentioned electro-optical modes, which have good viewing angle properties and improved response times, are increasingly used for LCDs for modern desktop monitors and even for displays for TV and for multimedia applications and thus are competing with the TN-LCDs.
Further to these displays, new display modes using cholesteric liquid crystals having a relatively short cholesteric pitch have been proposed for use in displays exploiting the so-called “flexoelectric” effect, which is described inter alia by Meyer et al., Liquid Crystals 1987, 58, 15; Chandrasekhar, “Liquid Crystals”, 2nd edition, Cambridge University Press (1992); and P. G. deGennes et al., “The Physics of Liquid Crystals”, 2nd edition, Oxford Science Publications (1995).
Displays exploiting flexoelectric effect are generally characterized by fast response times typically ranging from 500 μs to 3 ms and further feature excellent grey scale capabilities.
In these displays, the cholesteric liquid crystals are e.g. oriented in the “uniformly lying helix” arrangement (ULH), which also give this display mode its name. For this purpose, a chiral substance, which is mixed with a nematic material, induces a helical twist whilst transforming the material into a chiral nematic material, which is equivalent to a cholesteric material.
The uniform lying helix texture is realized using a chiral nematic liquid crystal with a short pitch, typically in the range from 0.2 μm to 2 μm, preferably of 1.5 μm or less, in particular of 1.0 μm or less, which is unidirectional aligned with its helical axis parallel to the substrates of a liquid crystal cell. In this configuration, the helical axis of the chiral nematic liquid crystal is equivalent to the optical axis of a birefringent plate.
If an electrical field is applied to this configuration normal to the helical axis, the optical axis is rotated in the plane of the cell, similar as the director of a ferroelectric liquid crystal rotate as in a surface stabilized ferroelectric liquid crystal display.
The field induces a splay bend structure in the director, which is accommodated by a tilt in the optical axis. The angle of the rotation of the axis is in first approximation directly and linearly proportional to the strength of the electrical field. The optical effect is best seen when the liquid crystal cell is placed between crossed polarizers with the optical axis in the unpowered state at an angle of 22.5° to the absorption axis of one of the polarizers. This angle of 22.5° is also the ideal angle of rotation of the electric field, as thus, by the inversion the electrical field, the optical axis is rotated by 45° and by appropriate selection of the relative orientations of the preferred direction of the axis of the helix, the absorption axis of the polarizer and the direction of the electric field, the optical axis can be switched from parallel to one polarizer to the center angle between both polarizers. The optimum contrast is then achieved when the total angle of the switching of the optical axis is 45°. In that case, the arrangement can be used as a switchable half wave plate, provided the optical retardation, i.e. the product of the effective birefringence of the liquid crystal and the cell gap, is selected to be the quarter of the wavelength. In this context, the wavelength referred to is 550 nm, the wavelength for which the sensitivity of the human eye is highest.
The angle of rotation of the optical axis (Φ) is given in good approximation by formula (1)tan Φ=ēP0E/(2πK)  (1)wherein    P0 is the undisturbed pitch of the cholesteric liquid crystal,    ē is the average [ē=½(esplay+ebend)] of the splay flexoelectric coefficient (esplay) and the bend flexoelectric coefficient (ebend),    E is the electrical field strength and    K is the average [K=½(k11+k33)] of the splay elastic constant (k11) and the bend elastic constant (K33)and whereinē/K is called the flexo-elastic ratio.
This angle of rotation is half the switching angle in a flexoelectric switching element.
The response time (τ) of this electro-optical effect is given in good approximation by formula (2)τ=[P0/(2π)]2·γ/K  (2)wherein    γ is the effective viscosity coefficient associated with the distortion of the helix.
There is a critical field (Ec) to unwind the helix, which can be obtained from equation (3)Ec=(π2/P0)·[k22/(ε0·Δε)]1/2  (3)wherein    k22 is the twist elastic constant,    ε0 is the permittivity of vacuum and    Δε is the dielectric anisotropy of the liquid crystal.
At higher electric fields, when the dielectric coupling becomes strong, the helix could be partially or completely unwound depending on the magnitude of the applied voltage. If the cholesteric liquid crystal possesses a positive dielectric anisotropy sD«.0d, the unwound state will be homeotropic and thus totally black when the cell is placed between crossed polarizers. The helix unwinding is a quadratic effect in contrast to the flexoelectro-optic effect which is a polar and linear effect. It should be noted that the helix unwinding by the applied electric field usually destroys irreversibly the ULH texture thus resulting in deterioration of the flexoelectro-optic mode of the device. In order to be practical, an electro-optic device based on the flexoelectrooptic effect must withstand a large temperature and field variation and still work functionally. This means, that such a device requires a stable ULH texture which after unwinding by the applied electric field, for instance, will be able to recover completely after switching off the field. The same should be valid for exposing the sample to high temperatures.
A further development are the so-called PS (polymer stabilised) displays. In these, a small amount of a polymerisable compound is added to the LC medium and, after introduction into the LC cell, is polymerised or crosslinked in situ, usually by UV photopolymerisation. The addition of polymerisable mesogenic or liquid-crystalline compounds, also known as “reactive mesogens” (RMs), to the LC mixture has proven particularly suitable in order to stabilize the ULH texture.
PS-ULH displays are described, for example, in WO 2005/072460 A2, U.S. Pat. No. 8,081,272 B2 or in Komitov et al. Appl. Phys. Lett. 2005, 86, 161118.
However, all attempts are connected with an increase of the operational voltage and a reduction of the switching speed.
Thus, one aim of the invention is to provide an alternative or preferably improved process of preparing a liquid crystal (LC) light modulation elements of the PS-ULH (polymer stabilised ULH) type, which does not have the drawbacks of the prior art, and preferably have the advantages mentioned above and below.
These advantages are amongst others favourable high switching angles, favorable fast response times, favorable low voltage required for addressing, compatible with common driving electronics, and finally, a favorable really dark “off state”, which should be achieved by an long term stable alignment of the ULH texture.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
Surprisingly, the inventors have found out that one or more of the above-defined aims can be achieved by providing process as defined in claim 1.